There are known effective heat-transporting devices, heat pipes in particular, consisting of sealed evacuated containers, mostly in metal, which are lined on the inside with a capillary material soaked with a fluid used as the heat-transfer agent.
An addition of heat to one end of the heat pipe causes the fluid to evaporate with the absorption of the heat of vaporization. The vapour so formed is induced by the pressure difference, no matter how small it may be, to flow towards the opposite end which is being cooled where condensation takes place, the heat of condensation being rejected into the surroundings through the wall of the pipe. The condensate absorbed by the capillary material reverses the direction of its flow due to the capillary pressure, returning into the zone of evaporation. The main equation describing the way the heat pipe is functioning is the pressure balance which may be thrown into the form EQU .DELTA.P.sub.c .gtoreq..DELTA.P.sub.f +.DELTA.P.sub.v ( 1)
where .DELTA.P.sub.c is the capillary pressure, N/m.sup.2 ; .DELTA.P.sub.f is the pressure difference in the fluid travelling through the capillary material, N/m.sup.2 ; .DELTA.P.sub.v is the pressure difference of the vapour in the vapour circuit, N/m.sup.2.
The pressure in capillary tubes of the cylindrical shape may be determined from Laplace's formula ##EQU1## where .delta. is the surface tension, N/m; .tau..sub.c is the radius of a capillary tube, m; .theta. is the wetting angle at the solid-fluid interface, deg.
The above formula holds if the fluid-vapour interface in the zone of condensing is flat. In the case of capillary passages which have an intricate shape, use is made of an equivalent radius instead of the radius of the capillary.
The pressure difference causing the flow of the fluid through a capillary path with a radius .tau..sub.c may be described by the formula ##EQU2## where G is the mass flow rate of the fluid, kg/s; .eta. is the dynamic viscosity, N.multidot.s/m.sup.2 ; L is the actual length of the heat pipe, m; .rho..sub.f is the density of fluid, kg/m.sup.3.
In the case of vapour flow, the pressure difference, .DELTA.P.sub.v, may be determined by the same formula, provided the flow is a laminar one. However, the vapor flow is commonly of the turbulent type which calls for using a much more complex formula to calculate the value of .DELTA.P.sub.v.
Equation (1) holds in the general case when the effect of the body forces on the heat-transfer agent in the heat pipe is negligible as, for example, when the pipe is oriented horizontally in the gravitational field and is of a small diameter.
When the heat pipe makes an angle .psi. with the horizontal, equation (1) must be supplemented by the term .+-..rho..sub.f gL sin .psi., where .rho..sub.f is the density of the fluid, kg/m.sup.3 ; g is the acceleration of the free fall, m/s.sup.2. Apparently, the additional term is used with the + sign when the zone of evaporating is located above the zone of condensing in the heat pipe and, as a consequence, the pressure loss therein appreciably increases with the increase in sin .psi. and the length L. Therefore, the performance of heat pipes--the distance of heat transporting and the heat flux transported--is low, especially in the region of the working temperatures of radio electronic equipment which are relatively low and call for using low-temperature heat-transfer agents with a low surface tension--a factor controlling the capillary pressure.
To obtain in this case a requisite value of .DELTA.P.sub.c, use must be made of capillaty materials with fine capillaries. However, it is evident from formula (3) that the friction losses increase directly as the radius of the capillary raised to the fourth power. All in all, the distance of heat transporting and the heat flux transported may grow so small when the flow of the heat-transfer agent is of a direction opposing the action of gravitational forces or any other body forces that the use of heat pipes may become questionable.
Known in the art is a heat pipe disclosed in U.S. Pat. No. 3,666,005. Said heat pipe comprises a number of sections connected to one another, each forming a heat pipe of its own. The section surfaces, including those at the ends of the heat pipe, are lined from the inside with a capillary material which is soaked with a heat-transfer agent. The sections are interlinked in such a way that the zone of condensing in a preceding section and the zone of evaporating in the succeeding one are separated by the same end face wall. Thus, the zone of condensing in a preceding section is in thermal contact with the zone of evaporating in the succeeding one. Since the heat-transfer agent circlates in each section independently of the other sections and the length of the section is comparatively short, the distance covered by the fluid agent making its path through the capillary material in each section is also short. Thus, this heat pipe is capable of transporting heat in an amount appreciably greater than the conventional heat pipe, using a capillary structure of a significant diameter, even if the gravitational forces are of a direction opposing the direction of the flow of the heat-transfer agent.
However, inherent in this pipe is a high heat resistance because of the heat exchange taking place between the sections due to heat conduction through the separating walls each having a heat resistance of its own.
Apparently, a requirement for a long heat pipe can be met by using a number of sections. Then, the total heat resistance of the heat pipe equalling the sum of the resistances of the separating walls will be high; in certain cases it may exceed the heat resistance of the conventional heat pipe. This will appreciably impair one of the assets of the known heat-transporting device, which is low heat resistance, so that the heat flux transported by the known heat pipe at a given temperature difference of the heat input and heat output will be lower than it is anticipated.
Efforts to increase the heat flux transported by a heat pipe through a reduction of the friction losses have materialized in a heat pipe disclosed in U.S. Pat. No. 3,543,839. The known heat pipe includes an evaporating chamber and a condensing chamber which contain capillary material and are interconnected by conduits into a closed air-tight circuit.
One of the conduits serves to convey vapour from the evaporating chamber, where it is formed due to a heat input, into the condensing chamber. Another conduit provides a liquid flow path for the condensate from the condensing chamber into the evaporating one, being filled to that end with a capillary material which is in contact with the same material contained in the chambers. A valve provided in the vapour flow path can control the friction loss in the conduit, functioning as a thermal switch which controls the heat flux in the heat pipe.
The heat pipe of said construction definitely cuts the losses of capillary pressure, for the liquid and vapour flows do not interact mechanically along the path of their travel. A thermal interaction of the two flows is practically also eliminated, which is a factor improving the thermodynamic characteristics of the heat pipe.
However, by analogy with the conventional heat pipes, the known pipe is not free from significant friction losses incurred due to the presence of the capillary material all the way down the conduit flowing over which is the condensate. Said losses appreciably reduce the distance of heat transporting and the heat flux transported if the heat pipe is so oriented in a field of body forces that the action of said forces or their components and the flow of the liquid heat-transfer agent are of opposite directions as this may be the case, for example, in the gravitational field when the evaporating chamber appears to be above the condensing one.
A further reduction of the friction loss along the liquid flow path has been achieved in the heat-transporting device disclosed in U.S.S.R. Inventor's Certificate No. 439,952.
The device includes an evaporating chamber containing a coaxially-arranged evaporator in a capillary material, which is in thermal contact with a source of heat, and a vapour-jet pump serving to transform the dynamic pressure of a heat-transfer agent in the vapour phase into the static pressure of the heat-transfer agent in the liquid phase. The evaporator has an axial bore with a transverse partition subdividing the evaporating chamber into two cavities, one containing the heat-transfer agent in the liquid phase and the other, in the vapour phase. A zone of a heat-exchanging chamber containing the heat-transfer agent in the liquid phase with a lower heat content is connected to a suction side of the vapour-jet pump by way of a first conduit and a zone of the heat-exchanging chamber containing the heat-transfer agent in the vapour phase with a higher heat content is connected to a discharge side of said pump and to the cavity of the evaporating chamber contained wherein is the heat-transfer agent in the liquid phase through a second conduit. A third conduit connects the cavity of the evaporating chamber containing the heat-transfer agent in the vapour phase to a nozzle of the vapour-jet pump. The capillary material of the evaporator is soaked with the liquid heat-transfer agent contained in one of the cavities of the evaporating chamber.
An addition of heat to the evaporating chamber results in the evaporation of the fluid the capillary material is soaked with. The vapour formed in the vapour cavity reaches the nozzle of the vapour-jet pump over the corresponding conduit. As the vapour is being issued from the nozzle, its dynamic head is transformed into the static pressure of the heat-transfer agent in the liquid phase, the pressure of the liquid phase at the discharge side increasing in excess of the pressure at the suction side at the same time. The so-called "pump effect" thus produced provides for an inflow of the heat-transfer agent with a lower heat content from the heat-exchanging chamber which causes condensation of the vapour emerging from the nozzle. The heat of condensation adds to the heat content of the heat-transfer agent which enters then the heat-exchanging chamber and the liquid phase cavity of the evaporating chamber.
Said device suffers from a number of significant drawbacks. Firstly, the heat-transfer agent heads for the surface of evaporation over the capillary material, the path being predominantly a longitudinal one. Therefore, any attempt to increase the length of the evaporator appears to be of no avail due to the same limitation as met with in the conventional heat pipe, which is capillary resistance. Secondly, difficulties are experienced in packaging the device due to the presence of an extra, third, conduit and the vapour-jet pump. Thirdly, the fact that the heat capacity of the inflow into the evaporating chamber is higher than that of the outflow from the heat-exchanging chamber causes a slight increase in the temperature of the vapour, as compared with the locale of heat rejection, and leads to a greater difference between the temperature of the locale of heat generation and that of the locale of heat rejection.